The invention relates generally to magnetic resonance imaging (MRI) and more particularly to a switched mode power supply (SMPS) that is compatible with MRI.
MRI systems use radio frequency (RF) detectors to amplify minute radio frequency signals emitted by atoms in a human or animal body after being excited by an RF pulse while being subjected to a constant magnetic field. The power distribution architecture found in conventional MRI systems has an intricate array of heavy gauge wires that supply the various voltage and current levels needed across the different MRI subsystems. This complexity results from the difficulty of providing efficient power conversion close to the physical location of the MRI system RF receivers in a way that does not affect the image quality.
FIG. 1 illustrates a power distribution architecture 10 associated with a scan room power supply that is known in the art. Highly sensitive amplifiers used throughout the receiver chain of an MR system require electrically clean supplies to avoid introducing conducted/radiated noise that can have an adverse effect in the quality of an image. Noise requirements have led to the use of linear regulators 12 in the scan room power supply (SRPS). Linear regulators can be inefficient but provide very good regulation, do not introduce high frequency noise, and have a very good transient response over wide load ranges. Present scan room power supplies take power from a conventional ac line and a three-phase line transformer that steps down the voltage and rectifies it to a lower dc bus voltage. The laminations used in the low frequency transformer are ferromagnetic and cannot be located in the immediate vicinity of the main magnet.
Large amounts of wasted heat must be removed from the active devices inside a SRPS using fans and/or heat sinks due to the inefficiency of linear power supplies. The size and weight of the thermal management system required and the need to maintain the line transformer away from strong magnetic fields have constrained the location of the SRPS to the equipment room.
The distance between the SRPS and the receivers is bridged by a complex wiring architecture can have wires of up to 20 meters long. Further complicating matters is the need to provide a point of load regulation that requires the use of 4 wires per output of the SRPS: a pair of cables for power delivery and a pair of thinner wired for voltage sensing. Losses in the long wires can be significant and efforts to reduce loss by the use of heavier cables come with an increase in the cost and weight of the system.
Although switched mode power supplies are known to deliver power efficiently, certain features associated with the use of conventional SMPS designs has prevented the use of such power supplies in MRI systems. These features include, for example, the fast turn on/off transitions across SMPS switching elements that are a well known source of EMI noise. If the noise has frequency components at frequencies within the imaging signal bandwidth, the quality of the image is adversely affected. Further, conventional SMPS designs employ energy storage devices the use ferrites. These ferromagnetic materials are not suitable for operation in the vicinity of the dc magnetic field surrounding the MRI system, since subjecting ferromagnetic materials to a large magnetic field can result in saturation and loss of their magnetic properties.
In view of the foregoing, it would be advantageous to provide a SMPS that operates at a switching frequency at which any potential EMI will have frequency components falling outside the imaging bandwidth of a corresponding MRI system. The SMPS energy storage components should be robust to large magnetic fields such that the SMPS can operate in close proximity to the magnet in an MRI system without experiencing adverse effects generally associated with conventional SMPS designs.